Photographic emulsions are comprised of a dispersing medium and silver halide microcrystals, commonly referred to as grains. As the grains are precipitated from an aqueous medium, a peptizer, usually a hydrophilic colloid, is adsorbed to the grain surfaces to prevent the grains from agglomerating. Subsequently binder is added to the emulsion and, after coating, the emulsion is dried. The peptizer and binder are collectively referred to as the photographic vehicle of an emulsion.
Gelatin and gelatin derivatives form both the peptizer and the major portion of the remainder of the vehicle in the overwhelming majority of silver halide photographic elements. An appreciation of gelatin is provided by this description contained in Mees The Theory of the Photographic Process, Revised Ed., Macmillan, 1951, pp. 48 and 49:
Gelatin is pre-eminently a substance with a history; its properties and its future behavior are intimately connected with its past. Gelatin is closely akin to glue. At the dawn of the Christian era, Pliny wrote, "Glue is cooked from the hides of bulls." It is described equally shortly by a present-day writer as "the dried down soup or consomm e of certain animal refuse." The process of glue making is age-old and consists essentially in boiling down hide clippings or bones of cattle and pigs. The filtered soup is allowed to cool and set to a jelly which, when cut and dried on nets, yields sheets of glue or gelatin, according to the selection of stock and the process of manufacture. In the preparation of glue, extraction is continued until the ultimate yield is obtained from the material; in the case of gelatin, however, the extraction is halted earlier and is carried out at lower temperatures, so that certain strongly adhesive but nonjelling constituents of glue are not present in gelatin. Glue is thus distinguished by its adhesive properties; gelatin by its cohesive properties, which favor the formation of strong jellies. PA0 Although collagen generally is the preponderant protein constituent in its tissue of origin, it is always associated with various "ground substances" such as noncollagen protein, mucopolysaccharides, polynucleic acid, and lipids. Their more or less complete removal is desirable in the preparation of photographic gelatin. PA0 (1) Photographic silver halide emulsion layers and other layers on photographic elements can contain various colloids alone or in combination as vehicles. Suitable hydrophilic materials include both naturally occurring substances such as proteins, protein derivatives, cellulose derivatives--e.g., cellulose esters, gelatin--e.g., alkali-treated gelatin (pigskin gelatin), gelatin derivatives--e.g., acetylated gelatin, phthalated gelatin and the like, polysaccharides such as dextran, gum arabic, zein, casein, pectin, collagen derivatives, collodion, agar-agar, arrowroot, albumin and the like . . . . PA0 Average aspect ratio EQU ECD/t&gt;5 (III) PA0 Average tabularity EQU ECD/t.sup.2 &gt;25 (IV) PA0 XI. Layers and layer arrangements PA0 XII. Features applicable only to color negative PA0 XIII. Features applicable only to color positive PA0 XIV. Scan facilitating features PA0 XV. Supports PA0 XVI. Exposure PA0 XVII. Physical development systems PA0 XVIII. Chemical development systems PA0 XIX. Development PA0 XX. Desilvering, washing, rinsing and stabilizing (post-development)
Photographic gelatin is generally made from selected clippings of calf hide and ears as well as cheek pieces and pates. Pigskin is used for the preparation of some gelatin, and larger quantities are made from bone. The actual substance in the skin furnishing the gelatin is collagen. It forms about 35 percent of the coria of fresh cattle hide. The corresponding tissue obtained from bone is termed ossein. The raw materials are selected not only for good structural quality but for freedom from bacterial decomposition. In preparation for the extraction, the dirt with loose flesh and blood is eliminated in a preliminary wash. The hair, fat, and much of the albuminous materials are removed by soaking the stock in limewater containing suspended lime. The free lime continues to rejuvenate the solution and keeps the bath at suitable alkalinity. This operation is followed by deliming with dilute acid, washing, and cooking to extract the gelatin. Several "cooks" are made at increasing temperatures, and usually the products of the last extractions are not employed for photographic gelatin. The crude gelatin solution is filtered, concentrated if necessary, cooled until it sets, cut up, and dried in slices. The residue, after extraction of the gelatin, consists chiefly of elastin and reticulin with some keratin and albumin.
Gelatin may also be made by an acid treatment of the stock without the use of lime. The stock is treated with dilute acid (pH 4.0) for one to two months and then washed thoroughly, and the gelatin is extracted. This gelatin differs in properties from gelatin made by treatment with lime.
In addition to the collagen and ossein sought to be extracted in the preparation of gelatin there are, of course, other materials entrained. For example, James The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, p. 51, states:
Superimposed on the complexity of composition is the variability of composition, attributable to the varied diets of the animals providing the starting materials. The most notorious example of this was provided by the forced suspension of manufacturing by the Eastman Dry Plate Company in 1882, ultimately attributed to a reduction in the sulfur content in a purchased batch of gelatin.
Considering the time, effort, complexity and expense involved in gelatin preparation, it is not surprising that research efforts have in the past been mounted to replace the gelatin used in photographic emulsions and other film layers. However, by 1970 any real expectation of finding a generally acceptable replacement for gelatin had been abandoned. A number of alternative materials have been identified as having peptizer utility, but none have found more than limited acceptance. Of these, cellulose derivatives are by far the most commonly named, although their use has been restricted by the insolubility of cellulosic materials and the extensive modifications required to provide peptizing utility.
Research Disclosure, Vol. 365, Sept. 1994, Item 36544, II. Vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda, A. Gelatin and hydrophilic colloid peptizers, paragraph (1) states:
This description is identical to that contained in Research Disclosure, Vol. 176, December 1978, Item 17643, IX. Vehicles and vehicle extenders, paragraph A.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power, both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of tabular grain populations in photographic emulsions.
In descriptions of these emulsions, as illustrated by Kofron et al U.S. Pat. No. 4,439,520, the vehicle disclosure of Research Disclosure Item 17643 was incorporated verbatim. Only gelatin peptizers were actually demonstrated in the Examples.
A difficulty that persisted throughout the 1980's was that no technique was known for preparing high chloride {100} tabular grain emulsions. It is known that high chloride emulsions possess performance advantages over high bromide emulsions in that high chloride emulsions (1) possess little, if any, native sensitivity to the visible spectrum, thereby allowing a more selective response when spectrally sensitized to a selected visible spectral region, (2) can be processed much more rapidly than high bromide emulsions, (3) require less frequent replenishment of processing solutions, and (4) pose less of a ecological burden than high bromide emulsions upon disposal.
The morphological instability of high chloride tabular grain emulsions with {111} major faces, the only emulsions available in the 1980's, limited the use of high chloride tabular grain emulsions. The recent discovery Of high chloride {100} tabular grain emulsions is illustrated by Maskasky U.S. Pat. Nos. 5,264,337 and 5,292,632, Szajewski U.S. Pat. No. 5,310,635, Brust et al U.S. Pat. No. 5,314,798, House et al U.S. Pat. No. 5,320,938, and Chang et al U.S. Pat. No. 5,413,904. While only gelatino-peptizers are actually demonstrated, all conventional peptizers of the types disclosed in Research Disclosure, Vol. 308, December 1989, Item 308119, Section IX, which is similar to Research Disclosure Item 17643, cited above, are stated to be useful.
Maskasky U.S. Pat. No. 5,284,744 taught the use of potato starch as a peptizer for the preparation of cubic (i.e., (100)) grain silver halide emulsions, noting that potato starch has a lower absorption, compared to gelatin, in the wavelength region of from 200 to 400 nm. Maskasky '744 does not disclose tabular grain emulsions.